Natural gypsum is also known as calcium sulfate dihydrate, terra alba or landplaster. Calcined gypsum is obtained by removing part of the water associated with the gypsum crystal. Synonymous terms for calcined gypsum are Plaster of Paris, stucco, calcium sulfate half-hydrate and calcium sulfate hemihydrate. Calcined gypsum, stucco and hemihydrate are the most commonly used terms, and they are used interchangeably in this application. When gypsum is mined, the natural rock is found in the dihydrate form, having about two water molecules associated with each molecule of calcium sulfate. In order to produce the stucco form, the gypsum can be calcined to drive off some of the water of hydration represented by the following equation:CaSO4.2H2O→CaSO4.1/2H2O+3/2H2O
Calcium sulfate hemihydrate is obtained by calcination to remove the associated water molecules. The hemihydrate is produced in at least two crystal forms. Alpha-calcined gypsum is made by a slurry process or a lump rock process whereby the calcium sulfate dihydrate is calcined under pressure. The alpha-calcined gypsum forms less acicular crystals than beta-calcined gypsum, allowing the crystals to pack tightly together, making a denser and stronger plaster. The crystal morphology of alpha hemihydrate allows water to flow easily between the crystals, requiring less water to form a flowable slurry. More elongated irregular shaped crystals are characteristic of the beta-hemihydrate, which is obtained by calcining gypsum at atmospheric pressure. This crystal structure results in a less dense product because the crystals are more loosely packed. The beta form also requires more water to fluidize the calcined gypsum. If the calcining of the dihydrate is performed at ambient pressure, the beta form is obtained and the cost is relatively low compared to the alpha-calcined gypsum.
A number of useful gypsum products can be made by mixing calcined gypsum with water and shaping the resulting gypsum slurry into a desired shape. The gypsum slurry is permitted to set by allowing calcium sulfate hemihydrate to react with sufficient water to convert the hemihydrate into a matrix of interlocking dihydrate crystals. As the matrix forms, the gypsum slurry becomes firm and holds the desired shape. Excess water must then be removed from the product by drying.
Set accelerators and set retarders (collectively known as “set modifiers”) are used in gypsum product compositions to control the set time of the gypsum product. If the set time is too long, contractors spend time waiting for the composition to set before they can move on to the next step of the project. When gypsum sets too quickly, the composition hardens before it is properly finished. In such cases, the surface may not be as smooth as desired or the product may not have been “worked” enough to produce a good finish.
Dry mixtures of calcium sulfate hemihydrate with additives can be pre-blended and produced. The dry mixtures are designed to be mixed with water to produce a high quality composition that is easy to use. One example of a setting mixture is one that produces a flooring slurry when combined with water, such as LEVELROCK® gypsum flooring material.
Set accelerators are utilized to hasten setting of the gypsum slurry. Retarders are added to setting mixtures to increase the working time of the gypsum slurry. The working time, also known as the open time, is the time that the slurry is pliable and can be shaped into a desired configuration.
Conventionally, proteinaceous retarders, such as SUMA retarder, and non-proteinaceous retarders, such as cream of tartar (potassium bitartrate), sodium citrate and diethylene-triamine-pentacetic acid and/or other organic chemicals/compounds known in the art, are utilized. The SUMA retarder, commercially available as a dry powder additive, has disadvantages associated with its use. For example, SUMA has a strong and offensive odor. It is proteinaceous, and sourced from hair and hooves of various animals, such as horses. SUMA also has undesirable aging characteristics. There is a long-standing need in the industry for a suitable replacement for this dry set retarder.
Diethylene-triamine-pentaacetic acid (DTPA) is also known as a set retarder, but the dry powdered forms have been found to be ineffective. For example, U.S. Pat. No. 4,661,161 to Jacacki (“Jacacki”) teaches the addition of a liquid form of diethylenetriamine pentaacetic acid (“DTPA”). U.S. Pat. No. 8,343,273 to Lettkeman et al., the entire disclose of which is incorporated herein by reference in its entirety, provides methods for utilizing DTPA as set retarder in compositions with beta stucco from calcined natural rock gypsum.
Dry powdered versions of various salts of DTPA have been found to have little or no effect on the reduction of water demand of calcined gypsums. Furthermore, the impact of dry versions of chelating agents have little effect on the setting action of calcined gypsums. Conventionally, various plasticizers such as naphthalene sulfonate and polycarboxylic acid, but not limited to the same, are used to decrease the amount of water (water demand) needed for preparing a workable gypsum slurry.
Synthetic gypsum, which is a byproduct of flue gas desulfurization processes from power plants, is also useful. Flue gas that includes sulfur dioxide is wet scrubbed with lime or limestone. Calcium from the lime combines with the sulfur dioxide to form calcium sulfite.CaCO3+SO2→CaSO3+CO2 Via forced oxidation, the calcium sulfite is converted to calcium sulfate.CaSO3+2H2O+½O2→CaSO4.2H2O
Synthetic gypsum is converted into calcium hemihydrate by calcination. While there are advantages to using synthetic calcined gypsum because this significantly decreases the environmental footprint, adding synthetic calcined gypsum usually increases water demand and more water is needed to prepare a workable gypsum slurry. Increasing the water demand is not desirable because this may affect unfavorably the strength of a gypsum product and may also increase the time and energy needed for evaporating the excess of water from the gypsum product.